To create realistic computer graphics images, surfaces of the graphics images should have surface detail. There are several methods that are conventionally used to create such surface details or textures. One common method of creating surface detail is to apply texture maps. Texture mapping refers to techniques for adding surface detail, or a texture map, to areas or surfaces of graphics images. A typical texture map is represented in a computer memory as a bitmap or other raster-based encoded format, and includes point elements, or “texels.” Generally, the process of texture mapping occurs by accessing the texels from the memory that stores the texture data, and transferring the texture data to predetermined points of surface being texture mapped. The texture map is applied according to the orientation and perspective of the surface on which the texture is applied. After texture mapping, a version of the texture image is visible on surfaces of the graphics image with the proper perspective and shading. Thus, the resulting graphics image appears to have the surface detail of the texture map.
Texture mapping creates realistic surface details in still graphics images, however, where the image is changing and moving, as in computer animation, texture mapping is unable to maintain the same level of realism as in a still image. That is, changes in the appearance of a surface, such as surface reflections of a surface having fine surface details and unevenness, are not produced where texture mapping is applied. What usually occurs is that any changes in the appearance of the graphics image due to a perspective shift are made for the entire surface of the graphics primitive to which the texture map is applied. Thus, the realism produced by texture mapping in a still graphics image is lost when applied in computer animation.
An alternative method of creating surface detail in a graphics image is to apply bump mapping. Bump mapping is a technique used in graphics applications for simulating the effect of light reflecting from small perturbations across a surface. See, Blinn, J. F., “Simulation of Wrinkled Surfaces,” Computer Graphics vol. 12 (August 1978). A bump map f(u, v) is interpreted as a height field that perturbs the surface along its normal vector at each point. However, rather than changing the surface geometry of the object to create the perturbations, only the normal vector for each pixel is modified. Thus, small surface details, represented by the individual pixels of a graphics image, can be realistically reproduced in computer animation applications. The conventional Blinn bump mapping technique computes the perturbed normal vector from the equation:N′=N+fu(Pv×N)+fv(Pu×N),Where N′ is the perturbed normal, N is the interpolated normal, fu and fv are the partial derivatives of the image height field, and Pu and Pv are the tangent vectors along the u and v axes, respectively.
Although bump mapping produces graphics images having more realistic surface details than texture mapping, implementing the equation to calculate a perturbed normal in a graphics processing system can be impractical and expensive. For example, complex circuitry is necessary to implement the aforementioned equation, calculating the perturbed normal on a pixel-by-pixel basis is a slow and resource intensive process, and including a circuit capable of performing the calculations consumes precious space in a graphics processing system. In applications where high integration of a graphics processing system is desirable, or where graphics images must be rendered quickly, as in computer animation applications, including circuitry in the graphics processing system capable of carrying out conventional methods of bump mapping is likely to be an unacceptable alternative.
Therefore, there is a need for method and apparatus that can provide surface detail on graphics images rendered by a graphics processing system that can maintain an acceptable level of realism in graphics animation applications, but does not require the circuitry necessary to carry out conventional methods of bump mapping.